This invention relates to imaging systems for electromagnetic radiation and, more particularly, to a system comprised of multiple reflecting elements having a real entrance pupil located in front of the imaging system.
Imaging systems for electromagnetic radiation, especially that portion of the radiation spectrum corresponding to visual and infrared radiation, are widely used. Of particular interest is the imaging of objects at long distances by telescope.
The optical elements of a telescope may be either refractive or reflective. The minimum number of such elements is generally recognized to be three, in order to provide the minimum number of parameters which are necessary to correct for spherical aberration, coma, astigmatism and field curvature. A telescope or imaging system which is comprised of three optical elements is generally known as a triplet.
One common type of triplet is constructed utilizing refractive optical elements, and is typically comprised of a negative lens interposed between two positive lenses.
The refracting triplet, although effective in controlling aberations and, thus, useful in many imaging applications, is not completely satisfactory for others. One significant disadvantage of the refracting triplet is that applications which require a large aperture are not readily accomodated. This disadvantage arises from the expense and difficulty encountered in accurately fabricating large lens elements. Such large lens elements also have a tendency to flex when pointed in various directions, such flexure resulting in an overall loss of image quality. Another disadvantage of the refracting type of imaging system is that the lens material may not be totally transparent to the wavelength or range of wavelengths of interest. Any absorption of the radiation of interest by the lens elements will naturally result in a degradation of image quality.
In order to overcome these disadvantages, it has been known in the prior art to use reflecting optical elements, such as mirrors, in order to focus the radiation to be observed. Reflecting optical elements are advantageous for many large aperture applications in that they are often less difficult to construct in large sizes than a corresponding refracting lens element. Also, reflecting elements can be made arbitrarily thick, therefore minimizing the problem of flexure. Also, a reflecting element can be made of materials which make the element lighter in weight than a corresponding refractive element, thus making them more suitable for airborne and outer space applications. Furthermore, reflective optical systems avoid the wavelength transparency problems of refractive optical elements.
Many prior art reflecting triplets are constructed such that light, entering the system from a distant object, first impinges on a primary mirror, is reflected onto a secondary mirror, then further reflected to a tertiary mirror and finally is focused to an image plane where an image of the distant object is formed.
A particular disadvantage of many prior art reflecting triplets arises from the arrangement of the optical elements. Typically, all three mirrors are aligned such that they lie on the optical axis of the optical system, resulting in their field of view also being along the optical axis. This arrangement results in the occlusion of a significant portion of the light entering the system from a distant object. This prior art arrangement of mirrors also results in a restriction of the field of view of the system and affects the power distribution between mirrors. Also, this design has the disadvantage that it is difficult to provide adequate baffling so that stray light can be prevented from reaching the image plane.
In order to overcome these disadvantages, it has been known in the prior art to construct a reflecting triplet optical system wherein the field of view is eccentic, that is, the field of view is not along the optical axis of the system but lies entirely to one side of it.
Representative of such an eccentric triplet optical system is U.S. Pat. No. 4,240,707, to W. Wetherell and D. Wemble, which patent is incorporated herein by reference. In the Wetherell patent there is described an all-reflecting, eccentric field, non-relayed optical system. This system is comprised of a reflective triplet having an aperture stop on the optical axis, the aperture stop being physically located on the secondary mirror. The entrance pupil of this prior art optical system is virtual, that is, it is located a large distance behind the optical system.
A disadvantage of such a prior art optical system arises because the entrance pupil is virtual, namely that a significant amount of beam wander will occur in front of the optical system. Such beam wander detrimentally affects the image quality in situations wherein the optical system must view through a small viewing port.
U.S. Pat. No. 4,265,510, assigned to the assignee of the present invention, which patent is herein incorporated by reference, describes an all-reflective, eccentric field, relayed, off-axis optical system having an entrance pupil in front of the primary mirror and an aperture stop positioned between the tertiary mirror and the image plane. Inasmuch as this is a relayed optical system, a field stop is positioned between the secondary and the tertiary mirrors at the point where an intermediate image is formed. This approach is particularly useful where the rejection of stray radiation is of great concern.
The real entrance disclosed in the aforementioned prior art, U.S. Pat. No. 4,265,510, acts to reduce beam wander, and, because the optical system is relayed, it has the advantage of allowing a field stop and aperture stop components to be contained within the optical system. However, for applications where the rejection of stray radiation is not of great concern, this prior art is subject to certain problems, among which the following are illustrative. The system tends to be large in size due to the size of its optical elements and the space required between them. It requires greater total optical power to form a pair of images, the first of which is located at the field stop and the second of which is re-imaged at the point where the image is sensed and, as a result of the greater total optical power needed, the system is subject to increased image quality degradation resulting from misalignments. The field of view is also limited in size.